The present invention relates to a process for the production of silicon carbide crystals and more particularly to the production of silicon carbide crystals having increased minority carrier lifetimes.
Semiconductor devices are increasingly required to accommodate high currents and/or high voltages without failing. Many high power applications call for the use of a semiconductor switch which is required to conduct a large current when turned on, and to block a high voltage when off. Although silicon (Si) has been the material of choice for many semiconductor applications, its fundamental electronic structure and characteristics prevent its utilization beyond certain parameters. Thus, interest for power devices has turned from silicon to other materials, including silicon carbide (SiC).
Silicon carbide has a variety of physical and electronic properties useful in semiconductor devices, including a wide bandgap, a high thermal conductivity, a low dielectric constant, high temperature stability, and high electric field breakdown. As a result, silicon carbide materials should theoretically allow production of electronic devices that can operate at higher temperatures, higher power and higher frequency than devices produced from silicon.
As an example, the wider bandgap of SiC as compared to silicon gives SiC a “critical electric field,” i.e., the peak electric field that a material can withstand without breaking down, which is an order of magnitude higher than that of silicon. This allows much higher doping and a much thinner drift layer for a given blocking voltage, resulting in a very low specific on-resistance in SiC-based devices. Although it has a much higher breakdown field than silicon, SiC has lower hole and electron mobilities and shorter minority carrier lifetimes, which can detrimentally affect the blocking voltage for a device as voltages increase.
Traditionally, two broad categories of techniques have been used for forming crystalline silicon carbide for semiconductor applications. One of these techniques epitaxially grows silicon carbide crystals by introducing suitable reactant gases into a system to form silicon carbide crystals upon an appropriate substrate. Epitaxially grown SiC crystals generally can exhibit minority carrier lifetimes suitable for various power device applications. As operating voltage demands for such power devices increase, for example, approaching 10 kilovolts (kV) and higher, the devices require increasingly thick silicon carbide layers to provide the requisite blocking voltage to prevent device failure. The production of suitably thick epitaxially grown SiC crystals, however, is not currently cost effective. Moreover, it can be undesirably time consuming to manufacture such crystals.
The other primary technique for the manufacture of SiC materials is sublimation growth, also referred to as physical vapor transport (PVT), in which a silicon carbide source material (typically a powder) is used as a starting material. The silicon carbide starting material is heated in a crucible until it sublimes. The sublimed material is encouraged to condense to produce the desired crystals. This can be accomplished by introducing a silicon carbide seed crystal into the crucible and heating it to a temperature less than the temperature of the source material. A pioneering patent that describes methods for forming crystalline silicon carbide for semiconductor applications using such seeded sublimation techniques is U.S. Pat. No. 4,866,005 to Davis et al., issued Sep. 12, 1989, which was reissued as U.S. Pat. No. Re. 34,861, issued Feb. 14, 1995, which patents are incorporated herein by reference as if set forth in their entirety.
Manufacturing SiC crystals using seeded sublimation techniques can offer cost and time advantages as compared to epitaxially growing SiC. As noted above, however, bulk SiC single crystals can have relatively short minority carrier lifetimes. As such, these materials typically are less suitable for use in certain applications, including power devices.
Thus there exists a need to produce silicon carbide crystals having increased minority carrier lifetimes, in a cost effective and time efficient manner, to facilitate large scale production of semiconductor devices including such materials